Adult mammalian Hg is a multimeric protein that includes two α and two β globin chains which together form the (α/β)2 tetrameric hemoglobin (Hb) molecule. Beta-thalassemias are a group of inherited blood disorders caused by a quantitative defect in the synthesis of the β chains of hemoglobin. In individuals with this disorder, the synthesis of β-globin chains is reduced or absent. Three main forms of the disease have been described: β-thalassemia major (β-TM or β0-TM) in which no β chain is produced, and β-thalassemia intermedia and β-thalassemia minor, in which β chain is produced but in lower than normal amounts. These conditions cause variable phenotypes ranging from severe anemia to clinically asymptomatic individuals. Individuals with β-TM usually present within the first two years of life with severe anemia, poor growth, and skeletal abnormalities during infancy. Affected children will require regular lifelong blood transfusions. β-thalassemia intermedia is less severe than β-thalassemia major and may require episodic blood transfusions. Transfusion-dependent patients will develop iron overload and require chelation therapy to remove the excess iron.
Despite extensive knowledge of the molecular defects causing β-thalassemia major (TM), less is known about the mechanisms responsible of the ineffective erythropoiesis. This latter is characterized by accelerated erythroid differentiation, apoptosis and maturation arrest at the polychromatophilic stage. It explains, at least in part, the profound anemia observed in this disease. Although it has been proposed that both the precipitation of unmatched globin chains as well as the accumulation of unbound iron could lead to oxidative stress and subsequent hemolysis, the mechanism by which apoptosis and maturation arrest are induced remains unclear.
The present inventors have elucidated the consequences of α chain cytoplasmic accumulation and the cascade of failed reactions that result therefrom which ultimately cause β-TM symptoms such as anemia. The discovery is based on the further clarification of the roles of the chaperone protein HSP70 and the erythrocyte maturation protein GATA-1 (https://ash.confex.com/ash/2012/webprogram/Paper48181.html). The inventors have discovered that HSP70 has important functions in both the cytoplasm and the nucleus of erythroblasts. A primary function of HSP70 in the nucleus is binding to the GATA-1 protein and preventing its cleavage and proteolytic degradation (by the protease caspase-3). HSP70 thus prevents inactivation of GATA-1 and preserves its function as a key factor in erythrocyte maturation. A secondary function of HSP70 is binding to α globin in the cytoplasm and ensuring that the protein chains are properly folded and can form tetrameric (α/β)2 Hb. Ordinarily, there is sufficient HSP70 available in the cell to carry out both of these functions. However, in β-TM cells, the HSP70 is monopolized by the excess of free α chains which accumulate in the cytoplasm. Thus, a disproportionate amount of the HSP70 is sequestered in the cytoplasm, and there is not sufficient HSP70 available for binding and protecting GATA-1 in the nucleus. Unprotected GATA-1 is proteolytically cleaved and inactivated, and proper erythrocyte maturation does not occur. Rather, the absence of active GATA-1 results in maturation arrest and apoptosis of immature erythrocytes at the polychromatophilic stage. This sequence of events is thus initially triggered by a lack of hemoglobin β chains and ultimately results in low (or no) erythrocyte production, causing anemia.